This invention relates to the solventless preparation of ester-substituted diaryl carbonates and in particular to a solventless method of making bis- methyl salicyl carbonate. In addition the present invention relates to a methods of preparing ester-substituted diaryl carbonates which minimize the use of organic solvents.
Ester-substituted diaryl carbonates such as bis-methyl salicyl carbonate have proven to be useful starting materials in the preparation of polycarbonates via the melt reaction of a diaryl carbonate with aromatic dihydroxy compounds. See for example, U.S. Pat. No. 4,323,668 in which rates of polymerization of bis-methyl salicyl carbonate with bisphenol A were shown to be higher than the corresponding rates of polymerization of bisphenol A with an unsubstituted diaryl carbonate, diphenyl carbonate. Notwithstanding the simplicity of its structure there are few reported preparations of ester-substituted diaryl carbonates.
A classical preparation of diaryl carbonates involves the reaction of a hydroxy aromatic compound such as phenol with phosgene gas in a two phase reaction system comprising water, an acid acceptor such as sodium hydroxide and a solvent such as methylene chloride or chloroform. Typical interfacial conditions used to prepare diphenyl carbonate (DPC) utilize water and methylene chloride phases, sodium hydroxide as a pH control measure and triethylamine as a catalyst. Under such conditions it is possible to convert phenol to DPC in essentially quantitative yield. Features of this process include the passage of phosgene gas into a reaction mixture comprising phenol and an organic solvent, and the removal of excess phosgene from the reaction mixture by passage of an inert gas such as nitrogen through the reaction mixture following completion of the phosgenation step. In both instances, solvent is entrained out of the reaction mixture by the flowing gases and must be trapped and recovered. Containment systems for preventing the escape of volatile organic compounds such as solvent emanating from reaction vessels frequently represent a significant cost of equipment used in chemical manufacturing.
Known methods for the preparation of diaryl carbonates such as diphenyl carbonate suffer in that their application to the preparation to ester-substituted diaryl carbonates results in only modest conversion of starting ester-substituted phenol to product ester-substituted diaryl carbonate and such known methods employ organic solvents during critical stages of the process where solvent containment is most difficult. The use of reaction systems which do not employ organic solvents to effect chemical transformations is desirable in that both environmental enhancements and reduction in manufacturing costs are made possible.
It would be desirable, therefore, to discover means for the efficient preparation of ester-substituted diaryl carbonates generally. In addition, it would be highly desirable to discover an efficient means of making ester-substituted diaryl carbonates from ester-substituted phenols and phosgene in a reaction system which avoids entirely or minimizes the use of organic solvents. Moreover, it would be desirable to use such a reaction system for the preparation of bis-methyl salicyl carbonate from methyl salicylate and phosgene.